Electric elevators conventionally have a car and a counterweight with these being connected by elevator ropes passing over a support sheave. The counterweight is selected of a weight between the empty and fully loaded weight of the car. Normal braking is accomplished by controlling the drive motor speed and torque to bring the car to a complete stop at the floor. Once at the floor, or near the floor for some control systems power is removed from the drive motor and a spring loaded friction brake is used to stop and hold or hold the car at the floor.
A safety is located on the car frame which engages the guide rails on downward overspeed of the car. Such engagement of the rails is not desirable in the upward direction because of the possibility of stopping with a greater than 1-G deceleration if the safety jams.
It is also known to prevent energization of the drive motor when the doors are open and the car is at a distance greater than a preselected distance from a landing. Some discrete movement is desirable to permit leveling of the elevator car, provided that the car is within close proximity of the landing with the doors open.
It is also possible, however, to experience an upward overspeed of the car. For instance this can occur with malfunction of the brake or control system and a lightly loaded car. This is particularly a problem when the car is at a low elevation so that substantial speed can be obtained by the time the car reaches the overhead building structure.
Movement from the floor can possibly occur even with the drive motor deenergized. Therefore, it is desirable to have a safety braking action to stop movement of the car beyond a predetermined distance with the doors open.
Certain elevator code regulations are in progress requiring a safety on upward overspeed and also on movement up or down beyond a specified distance with the elevator doors open. These codes will normally require that the safety braking system be independent of the regular controls.
A so called "Austrian safety", as illustrated in U.S. Pat. No. 4,538,706, has been successfully used on many elevators. This safety operates on the guide rail of the elevator and has found extensive use in smaller and low speed elevators. With large, high speed elevators, however, increased clearance for roller guide motion is required between the elevator car guides and the rail. Accordingly, the Austrian safety has not found success there because of the extensive clearance required. Since such Austrian safety is substantially a standard part, it would be desirable to have an arrangement wherein such safety could be applied to larger elevators.